Active-matrix-driven organic electro luminescence (or organic light-emitting diode (OLED)) display devices and active-matrix-driven liquid-crystal display devices include thin-film semiconductor devices referred to as thin film transistors (TFTs).
A display device of this type includes a TFT array device having TFTs arranged in an array. For each pixel, a TFT for driving the pixel (a driver transistor) and a TFT for selecting the pixel (a switching transistor) are formed.
Among transistors in a self-emitting OLED display device including an OLED device, the driver transistor and the switching transistor are required to be different from each other in performance capabilities. More specifically, the driver transistor is required to have excellent ON-state current characteristics to increase the drive performance of the OLED device whereas the switching transistor is required to have excellent OFF-state current characteristics.
The TFT includes a gate electrode, a semiconductor layer (a channel layer), a source electrode, and a drain electrode which are formed on a substrate. In general, a silicon thin film is used as the channel layer. Here, silicon thin films are roughly classified into a group of silicon thin films that are non-crystalline (amorphous silicon films) and a group of silicon thin films that have crystallinity (crystalline silicon thin films).
A TFT including a crystalline silicon thin film as a channel layer is higher in carrier mobility and more excellent in ON-state current characteristics, as compared with a TFT including an amorphous silicon thin film as a channel layer. Thus, a crystalline silicon thin film is known to be used as a channel layer of a driver transistor.
One of the conventional methods for forming a crystalline silicon thin film is to add a metal catalyst to an amorphous silicon film formed on a substrate to polycrystallize the amorphous silicon film by the application of heat. This method has the advantage of achieving crystallization at low temperature, but has the disadvantage of increasing the cost due to an increase in the number of processes and also has a difficulty in complete removal of metal elements after crystallization.
Moreover, another one of the conventional methods for forming a crystalline silicon thin film is to form a crystalline silicon thin film on a substrate by chemical vapor deposition (CVD). With this method, although the cost can be reduced because of a decrease in the number of processes, it is difficult to satisfy the aforementioned ON-state current characteristics required for the driver transistor since the resulting grain structure is a microlite structure.
Furthermore, another one of the conventional methods for forming a crystalline silicon thin film is to polycrystallize an amorphous silicon thin film by irradiating the amorphous silicon film with a beam of an excimer laser having a predetermined energy density at a predetermined film-formation temperature (Patent Literature (PTL) 1). With this method, however, the beam obtained as a result of the gas discharge in the vapor state is shaped. For this reason, the frequency with which the device needs an overhaul is high, and the running cost is accordingly high.
In addition, as an example of a method for crystallizing amorphous silicon using a laser, an amorphous silicon thin film is irradiated with a pulsed laser having a wavelength of 532 nm, to form a polycrystalline silicon thin film (PTL 2).